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Simulating Foreland Basin Response to Mountain Belt Kinematics And The Geological Society of America Memoir 212 2015 Simulating foreland basin response to mountain belt kinematics and climate change in the Eastern Cordillera and Subandes: An analysis of the Chaco foreland basin in southern Bolivia Todd M. Engelder Jon D. Pelletier* Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA ABSTRACT The relative importance of crustal thickening, lithospheric delamination, and cli- mate change in driving surface uplift and the associated changes in accommodation space and depositional facies in the adjacent foreland basin in the central Andes has been a topic of vigorous debate over the past decade. Interpretation of structural, geochemical, geomorphic, and geobiologic fi eld data has led to two proposed end- member Tertiary surface uplift scenarios for the Eastern Cordillera and Subandes in the vicinity of the Bolivian orocline. A “gradual uplift” model proposes that the rate of surface uplift has been relatively steady since deformation propagated into the Eastern Cordillera during the late Eocene. In this scenario, the mean elevation of the region was >2 km above mean sea level (msl) by the late Miocene or earlier. Alternatively, a “rapid uplift” model suggests that the mean elevation of the Alti- plano was <1 km above msl, and the peaks of the Eastern Cordillera were more than 2 km below their modern elevations until rapid uplift began ca. 10 Ma. Determin- ing which of these uplift scenarios is most consistent with the stratigraphic record is complicated by the potentially confounding effects of global climate changes and lithospheric delamination in the stratigraphic record. In this study, we use a coupled mountain-belt–sediment-transport model to predict the foreland basin stratigraphic response to these end-member surface uplift scenarios. Our model results indicate that the location and height of the migrating deformation front play the dominant roles in controlling changes in accommodation space and grain size within the fore- land basin. Changes in accommodation space and rates of sediment supply related to climate change and lithospheric delamination play secondary roles. Our results support the conclusion that the Eastern Cordillera likely gained most of its modern elevation prior to 10 Ma, in contrast with recent proposals that most of the modern elevation was obtained during the late Miocene. This conclusion is consistent with the most comprehensive paleoaltimetric analysis of the region to date. *[email protected] Engelder, T.M., and Pelletier, J.D., 2015, Simulating foreland basin response to mountain belt kinematics and climate change in the Eastern Cordillera and Sub- andes: An analysis of the Chaco foreland basin in southern Bolivia, in DeCelles, P.G., Ducea, M.N., Carrapa, B., and Kapp, P.A., eds., Geodynamics of a Cordil- leran Orogenic System: The Central Andes of Argentina and Northern Chile: Geological Society of America Memoir 212, p. 337–357, doi:10.1130/2015.1212(17). For permission to copy, contact [email protected]. © 2014 The Geological Society of America. All rights reserved. 337 Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/963891/mwr212-17.pdf by University of Arizona user on 27 August 2019 338 Engelder and Pelletier INTRODUCTION 2011b) as described further in the Discussion section. At a larger scale, the Andes as a whole have been an invaluable natural labo- Sediments eroded from mountain belts are primarily depos- ratory for exploring feedbacks between climate and rock uplift ited in foreland basins, which are elongate troughs created by (Montgomery et al., 2001; Strecker et al., 2007). fl exural loading of the lithosphere adjacent to mountain belts An unresolved issue for the central Andes is: When did the (DeCelles and Giles, 1996). Foreland basin stratigraphy can pro- topography of the central Andes rise to its modern elevation? One vide useful data for reconstructing climate and crustal deforma- model for the topographic development of the central Andes near tion histories within ancient mountain belts (e.g., Heller et al., the latitude of the Bolivian orocline posits that the mean topog- 1988; DeCelles et al., 1998; Marzo and Steel, 2000; Uba et al., raphy reached near-modern elevation following Neogene crustal 2007). The fl uvial systems that transport sediments from eroding thickening within the Subandean zone; thus, earlier crustal thick- mountain belts are sensitive to changes in sediment supply, dis- ening within the Eastern Cordillera was not suffi cient for the east- charge, and sediment accommodation rates, and thus, the foreland ern margin of the central Andes to rise to near-modern elevations basin stratigraphy in these systems is a partial record of changes (Isacks, 1988; Gubbels et al., 1993). In addition to crustal thick- in climate and the geometry of the mountain belt load through ening, continental lithospheric foundering has been proposed as a time. The fl exural profi le of foreland basin accommodation space mechanism for the generating at least some of the rapid Neogene depends on the rigidity of the lithosphere that is underthrusted surface uplift posited by this model (Kay and Kay, 1993; Garzi- beneath the mountain belt as well as the width and elevation of one et al., 2008; DeCelles et al., 2009). Continental lithospheric the mountain belt load. If the foreland basin geometry and rigid- foundering involves removal of negatively buoyant lower crust ity of the underthrusted lithosphere can be constrained, then it is (i.e., eclogite root) and mantle lithosphere by either delamination possible to infer changes in the mountain belt load that occurred or Rayleigh-Taylor instability. Initial paleoelevation and geomor- during the development of the foreland basin. As such, numer- phic studies did support late Miocene rapid surface uplift of the ous studies have inferred tectonic histories for ancient mountain region (Ghosh et al., 2006; Hoke et al., 2007; Garzione et al., belts by fi tting foreland basin geometries predicted by a coupled 2008). Pedogenic carbonate and carbonate cement samples col- mountain belt and foreland basin numerical model to observed lected between 17°S and 18°S within the Altiplano and Eastern foreland basin isopach data (e.g., Toth et al., 1996; Ford et al., Cordillera Neogene stratigraphy contain decreasing δ18O values 1999; Garcia-Castellanos et al., 2002; Prezzi et al., 2009). in progressively younger units (Garzione et al., 2008). Garzione The eastern margin of the central Andes (i.e., the portion of et al. (2008) interpreted this oxygen-isotope trend as evidence for the mountain belt that is to the east of the Altiplano Plateau) in an increase in elevation of the Eastern Cordillera of 2.5 ± 1 km southern Bolivia is an ideal region for constraining late Cenozoic from 10 to 7 Ma. Although these results are in agreement with changes in mountain belt geometry and climate through numeri- fossil leaf and clumped isotope data collected from the Altiplano cal modeling of foreland basin stratigraphy. The sediments pre- and Eastern Cordillera (e.g., Gregory-Wodzicki et al., 1998; served in the foreland basin have been well documented, and Ghosh et al., 2006), the elevation gain predicted by all three fi eld and laboratory constraints have been reported in the liter- methods can be complicated (i.e., biased toward larger values) by ature for shortening and exhumation rates in the hinterland for climate change due to the uplift of the Andes (Ehlers and Poulsen, the period of time over which the eastern margin of the central 2009). Also, recent studies have interpreted the change in Mio- Andes has developed. A W-E transect of the Cenozoic foreland cene oxygen isotopes to be the result of regional climate change basin stratigraphy is exposed within the retroarc fold-and-thrust caused by uplift of an already signifi cantly elevated (≥2 km) belt. Detailed isopach maps and stratigraphic sections for the central Andes above an orographic threshold (Poulsen et al., Cenozoic foreland basin stratigraphy have been developed based 2010; Insel et al., 2012). Most recently, Quade et al. (this volume) on a combination of measured sections from the fold-and-thrust showed that, while rapid uplift may have occurred locally in the belt and correlations between well-log data and two-dimensional Eastern Cordillera as documented by Garzione et al. (2008), the (2-D) seismic data (Sempere et al., 1997; DeCelles and Horton, easternmost Puna and Eastern Cordillera rose gradually to >3 km 2003; Echavarria et al., 2003; Uba et al., 2005, 2006). In addi- by no later than 15 Ma. Evidence for late Miocene rock uplift is tion to data from the foreland basin, the timing and magnitude of recorded by paleosurfaces that exist on both the eastern and west- shortening and exhumation within the mountain belt have been ern margins of the central Andes. Barke and Lamb (2006) esti- constrained by fi eld mapping and thermochronology (McQuar- mated 1.7 ± 0.7 km of localized rock uplift for the San Juan Del rie, 2002; Muller et al., 2002; Oncken et al., 2006; Ege et al., Oro surface of the Eastern Cordillera and Interandean tectono- 2007; Barnes et al., 2008, 2012). While most of the papers in morphic regions since 12–9 Ma, when the surface was abandoned this volume focus on the NW Argentina transect through the cen- and incised. The Barke and Lamb (2006) results do not constrain tral Andes, we focus here on the Bolivian transect due to these the magnitude of surface uplift, however. The difference between unusually rich data sets. We further assume that the two transects rock and surface uplift is particularly signifi cant because local- (Bolivia and NW Argentina) behave in a broadly similar way. ized rock uplift can be much higher than mean regional surface This assumption is supported by basin evolution and thermochro- uplift due to isostatic effects (England and Molnar, 1990).
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